<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>流程控制</title>
</head>
<body>
<script>
    let username='';
    if (!username){
      alert('用户名不能为空');
    }else{
      alert('欢迎'+username+'回归！');
    }
    let  gender = 0;
    switch (gender){
      case 1:
        console.log('男')
            break;
      case 0:
        console.log('女')
            break;
      default:
        console.log('未知')
    }
    //4.循环结构
    let names=['张三','李四','王五','赵六','钱七'];
    for(var i=0;i<names.length;i++){
      console.log(names[i])
    }
    console.log(i)

    //增强for循环
    for (let name in names){
      console.log(name)
    }

</script>
</body>
</html>